If add element with null value( map.put("test", null) wasAdded() and wasRemoved() in MapChangeListener return false
On remove this element from map (remove("teset")) listener will not be called.

Example:


import javafx.collections.FXCollections;
import javafx.collections.MapChangeListener;
import javafx.collections.MapChangeListener.Change;
import javafx.collections.ObservableMap;


public class TestObservableMap {
    public static void main(String[] args) {
        ObservableMap<String, Object> test = FXCollections.<String, Object>observableHashMap();
        test.addListener(new MapChangeListener<String, Object>(){

            public void onChanged(Change<? extends String, ? extends Object> change) {
                System.err.println("Is added: " + change.wasAdded());
                System.err.println("Is removed: " + change.wasRemoved());
            }

        });
        
        test.put("test", null);
        System.err.println(test.size());
        test.remove("test");
        System.err.println(test.size());
    }
}


Output of example:
Is added: false
Is removed: false
1
0